Many radio frequency oscillators today depend on the use of high dielectric coaxial resonators to establish the frequency of oscillation. These resonators are generally available off the shelf or can be customized to a particular frequency. The resonators are small, but relatively expensive, e.g. more than 20 times, in comparison with common surface mount parts such as resistors and capacitors. However, where high performance and stability are needed there are few alternatives.
In the case of a variable frequency oscillator, the frequency of the resonator needs to be pulled above or below the natural frequency of oscillation. This is normally done by shunting the resonator with a varactor diode, which changes in capacitance depending on the DC voltage at its terminals. Adding this component unfortunately lowers the performance, as the varactor is subject to more loss than the resonator. As the tuning range is made wider, the loading by the varactor becomes more significant, reducing the benefit of the original resonator in terms of its high performance and good phase noise performance. In short, the use of an expensive dielectric resonator becomes difficult to justify in a circuit where the quality factor, Q, is impaired by other components connected to it.
In these cases where the resonator Q of the oscillator becomes heavily dominated by the varactor or other tuning components, the use of a high Q resonator has little effect on the overall loaded Q.
In an attempt to overcome some of these problems, various alternative resonators have been proposed. For instance, U.S. Pat. No. 5,420,554 discloses a method and apparatus for adjusting a resonant frequency of a transmission line resonator assembly. The '554 patent uses laser tuning to center the resonant frequency. The tuning is accomplished by deflecting a metallized plastic assembly that is located directly atop a resonator electrode. However, the '554 patent does not make clear how deflection, and thus tuning, is made permanent. Further, the '554 device is fabricated on a printed circuit board rather than using thick film technology, which would allow for surface mounting in numerous applications.
In view of the above, there is a need for a low cost resonator that can offer performance that is nearly equivalent to the performance offered by a high Q coaxial dielectric design.